3D Medical implant designing and Printing - Perfect Adaptation to Surface with Complex Geometry
You need a shrinkwrap modifier on the plane, targeting the mandibule, then after that you use solidify modifier with 1mm thickness (turn on high quality normal too). Make sure your plane has plenty of geometry to shrinkwrap onto the mandible, do so by adding a subdivision modifier before the shrinkwrap if necessary.

If the shrinkwrap act weirdly try decimating the mandible a little bit with a decimate modifier since it's very high poly and might glitch the shrinkwrap.